Many shotgun shooters have found it desirable to be able to use several different sizes and types of cartridges in a single gun. To this end, some manufacturers have made available interchangeable barrels of different bores for particular guns. Such a system is not only bulky and expensive but inconvenient for quick conversion of the gun from one gauge to another.
It has also been suggested to provide full length barrel inserts bored to receive and fire smaller cartridges in the same gun. Aside from the cost of machining both the interior and exterior surfaces of such inserts additional means was needed for the extraction and ejection of fired shells from the gun. Such extraction and ejection was found especially difficult for pump action and autoloader type shotguns.